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Yan, Yong; Liu, Haocen; Liu, Chunyue; Zhao, Yuguo; Liu, Shuzhen; Wang, Dong; Fritz, Mathias; Ispas, Adriana; Bund, Andreas; Schaaf, Peter; Wang, Xiayan;
Efficient preparation of Ni-M (M = Fe, Co, Mo) bimetallic oxides layer on Ni nanorod arrays for electrocatalytic oxygen evolution. - In: Applied materials today. - Amsterdam [u.a.] : Elsevier, ISSN 2352-9407, Bd. 25 (2021)

Fabrication of economic and high-performance electrodes for electrocatalytic oxygen evolution reaction (OER) accounts for a crucial issue associated with developing powerful and practical water splitting systems. In this work, free-standing Ni/Ni-M (M = Fe, Co, Mo) bimetallic oxides core/shell nanorod arrays (Ni/Ni-M NRAs) were prepared through electroless deposition of transition metal species on black nickel sheet (nickel nanorod arrays (Ni NRAs)) followed by electrochemical oxidation. All three types of Ni/Ni-M NRAs demonstrated enhanced electrocatalytic activity toward oxygen evolution reactions (OER). Especially, Ni/Ni-Fe NRAs electrode exhibit small onset potential of 1.535 V at current density of 10 mA&hahog;cm^-2. In contrast, the OER durability of these three samples was distinct. At 500 mV constant overpotential, the current density loss in OER of Ni/Ni-Fe NRAs was merely 13.5% for a period of 20000 s; but Ni/Ni-Mo and Ni/Ni-Co NRAs had almost disappeared catalytic activity under the identical conditions. According to many reports, the results were different for the superior OER stability of Ni-based bimetallic catalysts. Electrochemical analysis revealed that the NRAs structure dramatically improves charge transfer efficiency and electrochemically active surface area (ECSA). The present study might provide a new insight to design and fabricate more practical and high-performance Ni-based electrodes for OER.

Herre, Patrick; Will, Johannes; Dierner, Martin; Wang, Dong; Yokosawa, Tadahiro; Zech, Tobias; Wu, Mingjian; Przybilla, Thomas; Romeis, Stefan; Unruh, Tobias; Peukert, Wolfgang; Spiecker, Erdmann;
Rapid fabrication and interface structure of highly faceted epitaxial Ni-Au solid solution nanoparticles on sapphire. - In: Acta materialia. - Amsterdam [u.a.] : Elsevier Science, ISSN 1359-6454, Bd. 220 (2021), S. 1-12

Supersaturated Ni-Au solid solution particles were synthesized by rapid solid-state dewetting of bilayer thin films deposited onto c-plane sapphire single-crystals. Rapid thermal annealing above the miscibility gap of the Ni-Au system followed by quenching to room temperature resulted in textured and faceted submicron-sized particles as a function of alloying content in the range of 0-28 at% Au. Morphologically, the observed kinetic crystal shapes are confined by close-packed planes; in addition, high-index facets are identified as a function of alloying content by TEM cross-sectioning and equilibrium crystal shape simulations. All samples exhibit a distinct <111> out-of-plane as well as in-plane texture along densely packed directions. Lattice parameters extracted from independent orthogonal X-ray and electron diffraction techniques prove the formation of a solid solution without tetragonal distortion imposed by the sapphire substrate. At the particle-substrate interface of highly alloyed particles segregation of Au atoms as well as dislocations in stand-off position are found. These observations are in-line with a semi-coherent interface, where Au segregation is triggered by the reduction of the overall strain energy due to: (i) a lower shear modulus on the particle side of the interface, (ii) the shifting of misfit dislocations in stand-off position further away from the stiffer substrate and (iii) a reduction of intrinsic misfit dislocation strain energy on the tensile side. In addition, the mechanical properties of pure and alloyed particles were characterized by in situ compression experiments in the SEM. Typical force-displacement data of defect-free single-crystals were obtained, reaching the theoretical strength of Ni for particles smaller than 400 nm. Alloying changes the mechanical response from an intermittent and discrete plastic flow behavior into a homogeneous deformation regime at large compressive strain.

Biele, Lukas; Schaaf, Peter; Schmid, Florian;
Specific electrical contact resistance of copper in resistance welding. - In: Physica status solidi. - Weinheim : Wiley-VCH, ISSN 1521-396X, Bd. 218 (2021), 19, S. 1-11

The electrical contact resistance (ECR) of copper (Cu-ETP R200, soft) contacts for resistance welding (RW) is characterized. ECR plays a major role in the RW process and provides local heat generation between the parts. A special determination method is used on different testing variants to observe the influence of contact pressure (two levels: 68, 155 MPa), contact temperature (20-550 &ring;C), and surface parameters, like roughness or oxide layer thickness, on the specific electrical contact resistance (SECR). For each surface parameter, three different levels are investigated. The study shows decreasing SECR with higher mechanical load on the contact and a more complex behavior for increase in contact temperature. SECR shows a characteristic behavior for contact states near the temperature-dependent tensile strength of the base material for rough and clean surfaces, where SECR approaches toward zero. The variation of oxide layer thickness and surface roughness has a strong influence on the resulting SECR and both surface parameters show a strong coupling regarding their effects.

Hergert, Germann; Wöste, Andreas; Vogelsang, Jan; Quenzel, Thomas; Wang, Dong; Groß, Petra; Lienau, Christoph;
Probing transient localized electromagnetic fields using low-energy point-projection electron microscopy. - In: ACS photonics. - Washington, DC : ACS, ISSN 2330-4022, Bd. 8 (2021), 9, S. 2573-2580

Low kinetic energy electrons are of interest for probing nanoscale dynamic processes using ultrafast electron microscopy techniques. Their low velocities reduce radiation doses and enhance the interaction with confined electromagnetic fields and, thus, may enable ultrafast spectroscopy of single nanostructures. Recent improvements in the spatial and temporal resolution of ultrafast, low-energy electron microscopy have been achieved by combining nanotip photoemitters and point-projection imaging schemes. Here, we use such an ultrafast point-projection electron microscope (UPEM) to analyze the interaction of low-energy electrons with transient electric fields created by photoemission from a nanogap antenna. By analyzing their kinetic energy distribution, we separate angular deflection due to radial field components from electron energy gain and loss due to their axial acceleration. Our measurements open up a route toward the spatial and temporal characterization of vectorial near-fields by low-energy electron streaking spectroscopy.

Li, Feitao; Oliva Ramírez, Manuel; Wang, Dong; Schaaf, Peter;
Formation and evolution of Au-SiOx heterostructures: from nanoflowers to nanosprouts. - In: Materials and design. - Amsterdam [u.a.] : Elsevier Science, ISSN 1873-4197, Bd. 209 (2021), S. 1-11
- Im Titel ist "x" tiefgestellt

This work reports the formation of circular cavities and Au-SiOx nanoflowers after annealing of thin Au film deposited on SiO2/Si substrates, and the transformation of nanoflowers to nanosprouts with increasing the annealing time. Two reference experiments indicate that both H2 and Si are indispensable for the above structures. The formation of cavities can be attributed to the SiO2 layer decomposition and the product, volatile SiO, provides a Si source for the formation of nanoflowers at the early stage. A model is proposed to indicate that SiO gas produced at the Si/SiO2 interface can diffuse to the surface assisted by the defects in the SiO2 layer before the decomposed cavities are exposed. Then the exposing of those cavities introduces another volatile SiO from the active oxidation of Si substrate, provoking a change in the direction of the main Si source, which in turn makes the one nanoparticle of the nanoflower split in two and finally form the nanosprout. The model about the escape of SiO further details SiO2 decomposition process, and the transformation mechanism from nanoflowers to nanosprout sheds light on a feasible nanofabrication method to design tunable size and shape of nanoparticles.

Schaaf, Peter; Constantinescu, Catalin; Matei, Andreea;
Preface on laser material interactions: from basic science to industrial applications (LaserMaterInter2020). - In: Applied surface science advances : an open access journal devoted to the physics and chemistry of surfaces and interfaces.. - Amsterdam : Elsevier, ISSN 2666-5239, Bd. 6 (2021), insges. 1 S.

Wang, Honglei; Cheng, Pengfei; Shi, Jun; Wang, Dong; Wang, Hongguang; Pezoldt, Jörg; Stich, Michael; Chen, Runfeng; Aken, Peter Antonie van; Huang, Wei; Schaaf, Peter;
Efficient fabrication of MoS2 nanocomposites by water-assisted exfoliation for nonvolatile memories. - In: Green chemistry : GC ; an international journal and green chemistry resource.. - Cambridge : RSC, ISSN 1463-9270, Bd. 23 (2021), 10, S. 3642-3648
- Im Titel ist "2" tiefgestellt

Efficient and green exfoliation of bulk MoS2 into few-layered nanosheets in the semiconducting hexagonal phase (2H-phase) remains a great challenge. Here, we developed a new method, water-assisted exfoliation (WAE), for the scalable synthesis of carboxylated chitosan (CC)/2H-MoS2 nanocomposites. With facile hand grinding of the CC powder, bulk MoS2 and water followed by conventional liquid-phase exfoliation in water, this method can not only efficiently exfoliate the 2H-MoS2 nanosheets, but also produce two-dimensional (2D) CC/2H-MoS2 nanocomposites. Interestingly, the intercalated CC in MoS2 nanosheets increases the interlayer spacing of 2H-MoS2 to serve as good candidates for the semiconductor devices. 2D CC/2H-MoS2 nanocomposites show superior electronic rectification effects in nonvolatile write-once-read-many-times memory (WORM) behavior with an ON/OFF ratio over 103, which can be rationally controlled by the weight ratios of CC and MoS2. These findings by the WAE method would open tremendous potential opportunities to prepare commercially available semiconducting 2D nanocomposites for promising high-performance device applications.

Wang, Anni; Gallino, Isabella; Riegler, Sascha Sebastian; Lin, Yi-Ting; Isaac, Nishchay A.; Sauni Camposano, Yesenia Haydee; Matthes, Sebastian; Flock, Dominik; Jacobs, Heiko O.; Yen, Hung-Wei; Schaaf, Peter;
Ultrafast formation of single phase B2 AlCoCrFeNi high entropy alloy films by reactive Ni/Al multilayers as heat source. - In: Materials and design. - Amsterdam [u.a.] : Elsevier Science, ISSN 1873-4197, Bd. 206 (2021), S. 1-12

High entropy alloy films of AlCoCrFeNi B2-ordered structure are formed during an ultrafast heating process by reactive Ni/Al multilayers. The self-propagating high-temperature reaction occurring in reactive Ni/Al multilayers after ignition represents an ultrafast heat source which is used for the transformation of a thin films Al/CoFe/CrNi multilayer structure into a single-phase high entropy alloy film. The materials design of the combined multilayers thus determines the phase formation. Conventional rapid thermal annealing transforms the multilayer into a film with multiple equilibrium phases. Ultrafast combustion synthesis produces films with ultrafine-grained single-phase B2-ordered compound alloy. The heating rates during the combustion synthesis are in the order of one million K/s, much higher than those of the rapid thermal annealing, which is about 7 K/s. The results are compared with differential scanning calorimetry experiments with heating rates ranging from about 100 K/s up to 25000 K/s. It is shown that the heating rate clearly determines the phase formation in the multilayers. The rapid kinetics of the combustion prevents long-range diffusion and promotes the run-away transformation. Thus, multilayer combustion synthesis using reactive Ni/Al multilayers as heat source represents a new pathway for the fabrication of single phase high-entropy alloy films.

Hotovy, Ivan; Rehacek, Vlastimil; Kemeny, Martin; Ondrejka, Peter; Kostic, Ivan; Mikolasek, Miroslav; Spieß, Lothar;
Preparation and gas-sensing properties of very thin sputtered NiO films. - In: Journal of electrical engineering. - Warsaw : De Gruyter Open, ISSN 1339-309X, Bd. 72 (2021), 1, S. 61-65

We present results on very thin NiO films which are able to detect 3 ppm of acetone, toluene and n-butyl acetate in synthetic air and to operate at 300&ring;C. NiO films with 25 and 50 nm thicknesses were prepared by dc reactive magnetron sputtering on alumina substrates previously coated by Pt layers as heater and as interdigitated electrodes. Annealed NiO films are indexed to the (fcc) crystalline structure of NiO and their calculated grain sizes are in the range from 22 to 27 nm. Surface morphology of the examined samples was influenced by a rough and compact granular structure of alumina substrate. Nanoporous NiO film is formed by an agglomeration of small grains with different shapes while they are created on every alumina grain.

Dulanto, Jorge; Sevillano-Bendezú, Miguel Ángel; Grieseler, Rolf; Guerra, Jorge A.; Korte, Lars; Dittrich, Thomas; Töfflinger, Jan Amaru;
Silicon interface passivation studied by modulated surface photovoltage spectroscopy. - In: Journal of physics. - Bristol : IOP Publ., ISSN 1742-6596, Bd. 1841 (2021), 1, S. 1-7

We demonstrate that the modulated surface photovoltage spectroscopy (modulated SPS) technique can be applied to investigate interface states in the bandgap, i.e. interface passivation, of crystalline silicon coated with a downshift layer such as hydrogenated aluminum nitride with embedded terbium ions by suppressing straylight with a cut-off filter. Different hydrogen contents influence the surface photovoltage spectra at photon energies below the bandgap of crystalline silicon. Modulated SPS reveals that at higher hydrogen content there is a lower signal and, thus, a lower density of surface defect states. Our experiments show that modulated SPS can become a powerful tool for characterizing defect states at interfaces which cannot be easily studied by other methods.